An atomic clock is an oscillator that provides a highly stable frequency over a long period of time because its resonance frequency is determined by the energy transition of atoms. In contrast, the frequency of a crystal oscillator is determined by the length of the crystal and is therefore much more susceptible to temperature variations than an atomic clock.
Atomic clocks are utilized in various systems that require extremely accurate and stable frequencies, such as in bistatic radars, GPS (global positioning system) and other navigation and positioning systems, as well as in various communications systems (e.g., cellular telephone systems).
In one type of atomic clock, a cell containing an active medium such as cesium (or rubidium) vapor. An optical pumping device, such as a laser diode transmits a light beam of a particular wavelength through the vapor, which is excited to a higher state. Absorption of the light in pumping the atoms of the vapor to the higher state is sensed by a photodetector which provides an output signal proportional to the light beam impinging on the detector.
By examining the output of the photodetector, a control system provides various control signals to ensure that the wavelength of the propagated light is precisely controlled.